Variable reactance transistor circuit



9, 1 H. c. GOODRICH, JR 2,570,938

' VARIABLE REACTANCE TRANSISTOR CIRCUIT Filed June 24, 1950 ATTORNEY tact area with the crystal.

Patented Oct. 9, 1951 UNITED STATES PATENT OFFICE VARIABLE REACT CIRCUIT Hunter C. Goodrich, Jn, Collingswood, N. 3., as

Signor to Radio Corporation of America, a corporation of Delaware Application June 24, 1950, Serial No. 170,088

13 Claims. 1

This invention relates generally to variable reactance electrical circuits, and has for its primary object to provide a semi-conductor circuit; for varying or modulating the resonant frequency of a tuned circuit.

Semi-conductor devices of the type comprising a semi-conducting body and a base, an emitter and a collector in contact with the body are presently well known. These devices are usually called transistors. The semi-conducting body may, for example, consist of a crystal of germanium or silicon treated in a manner well known in the art. The base electrode is in lowresistance contact with the crystal and may be a large-area electrode. The emitter and collector-electrodes are in rectifying contact with the crystal and may be point contacts or line contacts or they may even have a comparatively large con- It is also known that such a semi-conductor device may. be utilized in an amplifier or oscillator circuit.

In accordance with the present invention it has been found that a semi-conductor circuit may be arranged as a reactance circuit. Thus, a reactance appears between two of the electrodes of the transistor such as between the collector and base electrodes. The magnitude of the reactance may be adjusted or varied by a control voltage or sigiial. Such a variable reactance circuit may be utilized for many purposes such, for example, as for adjusting or varying the resonant frequency of a tuned circuit.

It is accordingly a further and important object of the present invention to provide a variable reactance circuit including a semi-conductor device.

linether object of the invention is to adjust or vary the width or the frequency of the pass band of a tuned amplifier with a variable semi-conductor reactance circuit.

A still further object of the invention is to adjust or modulate the frequency of a wave developed by an oscillator circuit by means of a semiconductor device connected in a reactance circuit.

In accordance with the present invention it has been found that the collector current of a semiconductor device of the type described may lag with respect to the emitter current. Such a lagging collector current represents a reactive impedance which appears looking into the collector and base electrodes. This reactive impedance may be adjusted or varied by adjusting or varying either the collector or the emitter bias voltage. Thus, a variable reactance circuit in accordance with the invention comprises a controlled circuit such as a resonant circuit which is coupled between the collector and base electrodes of a transistor. A path is provided between the emitter and base electrodes which presents a low impedance to currents at the frequency of the resonant circuit. By varying either the emitter or collector bias voltage the reactive impedance presented by the semi-conductor device is varied. Since this reactive impedance is coupled across the resonant circuit the frequency of the resonant circuit is also varied. v

, The resonant circuit may, for example, be the output circuit of an amplifier. In this case, the frequency of the pass band of the amplifier is varied. Alternatively, another tuned circuit may be coupled to the first resonant circuit and in that case the width of the pass band of the amplifier is varied. Finally, the resonant circuit may be the frequency determining circuit of an oscillator. In such a case, the frequency of the wave developed by the oscillator may be adjusted or modulated in accordance with the signal.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, in

which: I

Figure 1 is a circuit diagram of a variable semiconductor reactance circuit embodying the present invention;

Figure 2 is an equivalent circuit diagram of the reactance circuit of Figure 1;

Figure 3 is a circuit-diagram of a variable reactance circuit in accordance with the invention arranged for varying the width of the band pass of an amplifier or the frequency of its pass band; and

Figure 4 is a circuit diagram of an oscillator circuit associated with a variable reactance circuit in accordance with the invention, for varying or modulating the frequency of the wave devel- 3 r per-factions as commonly employed for best results in crystal rectifiers or transistors. Body ii may consist of germanium which may be prepared so as to be an N type semi-conductor as is well known. The surface of body ii may be polished and etched in a conventional manner.

Base electrode l2, emitter electrode I! and collector electrode N are in contact with body It. As pointed out previously, base electrode I2 is in low-resistance contact with body H. and

7 may, for example, consist of a suitable piece of metal such as biaSB which is soldered to body I I. Emitter electrode 13 and collector electrode [4 are in rectifying contact with body II and may consist of point electrodes, they may be line contact electrodes or they may even be in large-area contact with the body ll. Electrodes l3 and I may consist of fine pointed phosphor bronze. s

For operation as an amplifier a bias voltage in the forward direction must be applied between emitter l3 and base l2. Furthermore, a bias voltage in the reverse direction must be applied between collector It and base i2. Assuming that body ii consists of an N type crystal emitter I! should be positive with respect to base I! while collector ll should be negative with respect to the base. If the crystal is of the P type the potentials must be reversed. Thus; battery l5 may circuit of Figure 1 has been illiiitrated in Figure 2. It'will be seen that rwonant circuit 2i consisting of inductor 22 afid'capflcitor II is shunted I by variable inductor ii. Inductor 25 indicates iusted toproduce an emitter current of .75 ma.,

wires of tungsten or is provided for the purpose of applying a voltage in the reverse direction to collector II. To this end the positive terminal of battery 2| is grounded while its negative terminal is connected to collector I through a controlled circuit 2! which may be a tuned circuit. Tuned circuit it includes inductor 22 and capacitor 23 connected in parallel. Bypass capacitor 'may be connected across battery 2|.

Capacitor it connected between emitter l3 and base I! presentsa low impedance to currents at the frequency of resonant circuit 2|. More specifically, the reactive impedance of capacitor II at the frequency of resonant circuit 2! should be low compared to the equivalent emitter resistance, that is, the resistance which appears lookin into emitter electrode l3. emitter resistance of device II is of the order of a few hundred ohms and usually less than 500 ohms.

It has been found 1 when operated as shown with a low impedance path betweenemitter 13 and base I! for currents at the frequency of resonant'circuit 2| produces a lagging reactive collector current, that is, the collector current lags with respect to the emitter current. This lagging collector current raises the apparent resonant frequency of tuned circuit II. It has also beenfound that the magnitude of this reactive effect is a function of either the emitter or the collector bias voltages. Thus, by varying tap IT on resistor ii or by adjusting battery Zll the-effective resonant frequency of circuit 21 may be adjusted or varied. An equivalent circuit diagram of the reactive The equivalent that the circuit of Figure the apparent resonant frequency of turned circuit 2i isnowraisedto 645 kc. Thus,anincrease oftheemittercurrentcausesanincreaseofthe apparent resonant frequency of tuned circuit 2! caused by a decrease of the inductance of inductor 2i.

4 Figure 3 to which reference is now made, shows an amplifier having a pas band of adjustable frequency or of adjustable width by means of the variable rectance circuit of the invmtion. The variable reactance circuit semiconductor device ll. Emitter electrode i3 is connected to battery I! through resistor ll. Col-'- lector electrode ll is connected to battery II through-collector resistor 3i. Base electrode [2 is again grounded and capacitor it connect emitter electrode I: to base electrode [2, A control signal source indicated at 32 has one terminal grounded while its other terminal 8 coupled to emitter I: throu h coupling capacitor 33. Capacitor 33 is optional and may be omitted ifthesignaldevelopedbysourceflisadircet current voltage.

The circuit described herein operates in substantially the samemannerastbecircuitof Figure 1. Capacitor it should again proviik a low impedance path for currents at thefrcquemy of resonant circuit 2i which is coupled to collector electrode ll through coupling capacitor 14. However, capacitor ll should have an appreciable impedance for signal currents developed by source '32. The resistance of resistor 31 should be large compared to the equivalent collector resistance which is the 'ce looking into collector electrode ll capacitor 3 should have a capacitance which is large compared to the capacitance of capacitor 2! of resonant circuit 21. I

Resonant circuit it may, for example, form the output load of an amplifier circuit including vacuum tube tetrode 3E. The cathode of amplifier tubelimaybegroundedandflmsfinalto be amplified may be impressed by input terminals I! and coupling capacitor ll between the is coupled to the plate of amplifier tube Ii through coupling capacitor 41.

In accordance with the control signal devcl-' opedby source I! the'resonantfrequencyof tuned circuit 2! lsvariedor modulated thereby to vary the frequency of the passband. Alternatively, the output signal may be derived from tuned cutput'circuit n which has a flxedfre- Furthermom,couplingv quency equal to the normal frequency of tuned circuit 2! and which may be inductively coupled to tuned circuit 2|. In that case, the width of the pass band of amplifier tube 36 may be adjusted or varied by the control signal developed by source 32. It will be obvious that the width of the amplifier pass band depends on the difference between the apparent resonant frequencies of tuned circuits 2! and 44.

It is also feasibl to adjust or modulate the frequency of the wave developed by an oscillator circuit. Such a circuit has been illustrated in Figure 4. The variable reactance circuit of Figure 4 is identical to that of Figure 3 and therefore'a further description is not necessary here.

- Resonant circuit 2| which is coupled by coupling capacitor II to collector electrode I4 is now the frequency-determining circuit or the tank circuit of the oscillator. The oscillator includes vacuum tube triode 58 having its cathode grounded. Resonant circuit 2| is connected to the anode of tube 50. The other terminal of resonant circuit 2| is connected to a .suitable anode voltage supply indicated at +3 which may be bypassed to ground by capacitor 5 I. Inductor 52 is inductively coupled to inductor 22 of. resonant circuit 2|. One terminal of inductor 52 is grounded while its other terminal is coupled to the control grid of oscillator tube 50 through coupling capacitor 53. The control grid of oscillator tube 5|! may be grounded through grid leak resistor 54.

The oscillator circuit is a tuned-plate oscillator which functions in a conventional manner. The frequency of the wave developed in tank circuit 2| may be adjusted or modulated in accordance with the signal impressed on terminals 32. If this signal is an automatic frequency control voltage, coupling capacitor 33 should be omitted. Thus, the circuit of Figure 4 may be used either in the manner of an automatic frequency control circuit or as a frequency modulator. The output wave may be obtained from inductor 55 inductively coupled to inductor 22.

There has thus been disclosed a variable reactance transistor circuit which may be used for adjusting, varying or modulating the frequency of a tuned circuit. Such an arrangement may be utilized for adjusting or modulatingthe frequency or the width of the pass band of an amplifier. Alternatively, the frequency of the wave developedby an oscillator circuit may be adjusted or modulated. The reactance of the variable reactance circuit is a function of the emitter and collector bias voltages. Preferably, however, the emitter bias voltage is adjusted or varied to vary the effective reactance of a circuit.

What I claim is:

l. A variable reactance circuit comprising a semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode incontact with said body, means for applying a bias voltage in the forward direction between said emitter and base electrodes and for applying a bias voltage in the reverse direction between ,said collector and base electrodes, a controlled resonant circuit coupled between said collector and base electrodes, an impedance element connected directly between said emitter and base electrodes and presenting a low impedance to currents at the frequency of said resonant circuit to provide an electrical low-impedance path for said currents between said emitter and base electrodes, and means for varying one 0! said bias volta es.

6 thereby to vary the reactive impedance presented by said device and appearing between said collector and base electrodes.

2. A variable reactance circuit comprising a semi-conductor device including asemi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means for applying a bias voltage in the forward direction between said emitter and base electrodes and for applying a bias voltage in the reverse direction between said collector and base electrodes, a resonant circuit coupled between said collector and base electrodes, a capacitor connected between said emitter and base electrodes and presenting a low impedance to currents at the frequency of said resonant circuit, and means for varying one of said bias voltages, thereby to vary the reactive impedance presented by said device and coupled to said resonant circuit.

3. A variable reactance circuit comprising a semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means connecting said base electrode to a point of substantially fixed potential, means including a source of voltage for applying a voltage in the forward direction between said emitter and base electrodes and for applying a voltage in th reverse direction between said collector and base electrodes, a resonant circuit coupled between said collector electrode and a point of substantially fixed potential, a capacitor connected between said emitter and base electrodes and providing a low impedance path between said emitter and base electrodes for currents at the frequency of said resonant circuit, and means for varying one of said voltages, thereby to vary the resonant frequency of said resonant circuit.

4. A variable reactance circuit comprising a semi-conductor device including a semi-conducting body, a base electrode. an emitter electrode and a collector electrode in contact with said body, means connecting said base electrode to a point of substantially fixed potentiaL'means including a source of voltage for applying a voltage in the forward direction between said emitter and base electrodes and for applying a voltage in the reverse direction between said collector and base electrodes, a resonant'circuit coupled between said collector electrode and a point of substantially fixed potential, a capacitor connected between said emitter and base electrodes and providing a low impedance path between said emitter and base electrodes for currents at the frequency of said resonant circuit, and a source of signal for varying the voltage applied between said emitter and base electrodes, thereby to vary the resonant frequency of said resonant circuit.

5. A variable reactance circuit comprising a semi-conductor device including a semi-conducting body. a base electrode, an emitter electrode and a collector electrode in contact with said body, means connecting said base electrode to a point of substantially fixed potential, means including a source of voltage for applying a, voltage in the forward direction between said emitter and base electrodes and for applying a voltage in the reverse direction between said collector and base electrodes, a resonant circuit including a first capacitor, a second capacitor coupling said resonant circuit between said collector electrode and a point of substantially fixed potential, the reactive impedance of said third capacitor at the frequency at said resonant circuit being small a' collector electrode in contact with said body.

means connecting said base electrode to a point of substantially fixed potential, means for applyihg a voltage in the forward direction between said emitter and base electrodes, a source of voltage and a resistor connected in series between said collector and base electrodesfor applying a voltage in the reverse direction between said collector and base electrodes the resistance of said resistor being large compared to the resistance which appears looking into said collector electrode, a resonant circuit including a first capacitor, a second capacitor coupling said resonant circuit between said collector electrode and a point of substantially fixed potential, the capaci'tance of said second capacitor being large comto that of said first capacitor, a third capactor connected between said emitter and base electrodes, the reactive impedance of said third capacitor at the frequency'of said resonant circuit being small compared to the resistance which aplooking into said emitter electrode, and a source of signal for varyin the voltage applied fcircuit.

7. A variable reactance circuit comprising an Qamplifier stage having input and output terlminals, a resonant circuit coupled to said output iterminals, a semi-conductor device including a semi-conducting body, a base electrode, an emit- ;terelectrode and a collector electrode in contact mm said body, means connecting said base electrode to a point of substantially fixed potential, means including a source of voltage forapplying 'a voltage in the forward direction between said emitter and base electrodes and for applying a voltage in the reverse direction between said collector and base electrodes, a first capacitor coupling said resonant circuit to said collector electrade, a second capacitor connected between said emitter and base electrodes and providing a low impedance path between said emitter and base electrodes for currents at the frequency of said resonant circuit, and a source of signal for varying the voltage applied between said emitter and base electrodes, thereby to vary the resonant frequency of said resonant circuit,

8. A variable reactance circuit comprising an amplifier stage having input and output ter- '-minals, a parallel resonant circuit coupled to said output terminals and including a first capacitor, a semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means connecting said base electrode -to a point of substantially fixed potential, means including a source of voltage for applying a voltage in the forward direction between said emitter and base electrodes and for applying a voltage in the reverse direction between said collector and base electrodes, a second capacitor coupling said resonant circuit to said collector electrode, the

capacitance of said second capacitor being large .between said emitter and base electrodes, thereby 'to vary the resonant frequency of said resonant compared tothatof said first WW1. tthird capacitor connected between said emitter and base electrodes the reactive impedance of said third capacitor at the frequency of said resonant circuit being small compared to the resistance which appears looking into said emitter electrode, and a source of signal for, varying the voltage applied between said emitter and base electrodes. thereby to, vary the resonant frequency ofsaid resonantcircuit. Y

9. A circuit having a variable band width comprising an amplifier. stage having input and output terminals, a first resonant circuit coupled to said output terminals, a second resonantcircuit inductively coupled Eto said first resonant circuit,

said resonant circuits beingnormally-tlmed to substantially the same frequency, a semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means connecting said base electrode to a point stantially, fixed potential, means including'--a source of voltage for applyinga voltage in the forward direction between said base electrodes and for applying a voltage'in the re? verse direction between ,said collector-fund base electrodes, a first capacitor coupling said first resonant circuit to said collector electrode, a second capacitor connected between said emitter and base electrodes and providinga low impedancepath between said emitter and base electrodes for currents at the frequency of said firstresonant circuit, a source of control signal for varying thewith said body, means connecting said-base electrode to a point of substantially fixed potential, means including a source of voltage for applying a voltage in the forward direction betweensaid emitter and base electrodes and for applyingiil, voltage in the reverse direction between said col-' lector and base electrodes, a first capacitor coupling said frequency determining circuit to said collector electrode, a second capacitor connected between said emitter and baseelectrodes and v providing a low impedance path between said emitter and base electrodes for currents at the frequency of said frequency determining circuit, and a source of signal for varying the voltage applied between said emitter and base electrodes. thereby to vary the resonant frequency of said oscillator circuit.

11. A frequency modulation system an oscillator circuit including a frequency de'termining resonant circuit having a first capacitor,

a semi-conductor device including a semi-oonducting'body, a base electrode, an emitter electrode and a collector electrode in contact with said body,-means connecting saidbase electrode.

and base electrodes, the reactive impedance of said third capacitor at the frequency of said resonant circuit being small compared to the resistance which appearslooking into said emitter electrode, and a source of modulation signal for varying the voltage applied to said emitter and base electrodes, thereby to modulate the resonant frequency of said oscillator circuit.

12. A frequency modulation system comprising an oscillator circuit including a frequency determining resonant circuit having a first capacitor, 2. semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means connecting said base electrode to a point of substantially fixed potential, means for applying a voltage in the forward direction between said emitter and base electrodes, a source of voltage and a resistorconnected in series bepath between said emitter and base electrodes for currents at the frequency of said resonant circuit, and a source of modulation signal for varying the voltage applied to said emitter and base electrodes, thereby to modulate the resonant frequency of said oscillator circuit.

13. A frequency modulation system comprising an oscillator circuit including a frequency-determining resonant circuit having a first capacitor, 2, semi-conductor device including a semi-conducting body, a base electrode, an emitter electrode and a collector electrode in contact with said body, means connecting said beam electrode to a point of substantially fixed potential, means for applying a voltage in the forward direction between said emitter and base electrodes, .a source of voltage anda resistor connected in series between said collector and base electrodes for ap-" plying a voltage in the reverse direction between said collector and base electrodes, the resistance of said resistor being large compared to the resistance which appears looking into said collector electrode, a second capacitor for coupling said resonant circuit to said collector electrode, the capacitance of said second capacitor being large compared to that of said first capacitor, a third capacitor connected between said emitter and base electrodes, the reactive impedance of said third capacitor at the frequency of said resonant circuit being small compared to the resistance which appears looking into said emitter electrode,

' and a source of modulation signal for varying the REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,486,776 Barney Nov. 1, 1949 2,533,001 Eberhard Dec. 5, 1950 

